Fiber optic technology uses glass or plastic threads, called ‘fibers’ to transmit data. A fiber optic cable consists of a bundle of fibers, each of which is capable of transmitting messages modulated onto light waves. Dense Wavelength Division Multiplexing (DWDM) is an optical technology that works by combining and transmitting multiple signals simultaneously at different wavelengths (or channels) on the same fiber. In effect, one fiber is transformed into multiple virtual fibers.
DWDM networks strive to deliver each signal to a receiver with a satisfactory Optical Signal to Noise Ratio (OSNR) to achieve the desired system Bit Error Ratio (BER). Thus, the noise incurred by a signal as it traverses the network from transmitter should not exceed a predetermined value, and degradations to the signal, in terms of gains or losses, need to be controlled. However, as wavelengths pass through different components from a transmitter to a receiver, gain or loss of power (due to ripple) causes wavelengths to incur different amounts of noise, because the wavelength gains or loses power as it traverses amplifiers in its path. The amount of optical noise in each channel will differ depending upon the number of devices encountered by each signal in its path from transmitter to receiver, due to gain variations between channels. Some gain variations are systematic variations; for example the gain ripple of an amplifier is substantially a function of wavelength, and so accumulates along a chain of amplifiers from transmitter to endpoint. Other gain variations are random, for example, loss variation of individual components of demux/mux filter structures will be seen a loss variation between the different paths through them.
The problem of differing OSNR on different channels is further exacerbated in ring based architectures such as those generally used in metro applications. In ring architectures, individual wavelengths or bands of wavelengths travel different distances around the ring between their sources and destinations, resulting in different channels encountering different numbers of amplifiers and filters as they traverse their path.
When designing optical transport systems, a link budget for a given communication path is selected, wherein the link budget is the aggregate permissible OSNR (or BER) for each of the channels on a given fiber. Each component in the path introduces a gain or loss ripple to the wavelength that affects the available link budget for the transmission. In particular, amplifiers, which are often needed to boost signal power during transmission, also amplify the noise of a signal and introduce new noise, and therefore degrade the OSNR of a wavelength. Typical link budgets of some systems allow no more than three times the standard deviation from the original power for the wavelength, and as a result the worst case ripple through the system for a signal is three times the standard deviation from the original value. However, the addition of components can rapidly cause the link budge to be exceeded. Link budget impairments caused by wavelength dependent loss may be removed by re-generating the wavelength at periodic intervals in the transmission path using signal regenerators. Sufficient signal regenerator components are added in the communication path to ensure that the resultant gain or loss caused by ripple is within a predetermined deviation from the average. One problem with such an arrangement, however, is that signal regeneration is expensive, and thus it is desirable to minimize the number of signal regenerators in a transmission path. Thus, the need for amplifiers to provide adequate signal power causes the system to require wavelength power “balancing” to ensure that the signal goes end to end with acceptable BER (or OSNR). However, introduction of power balancing in such a solution is often not desirable in metro networks, because introducing overhead into a link solely to assist in power balancing, but otherwise providing no functional value, undesirably increases the cost of a cost-sensitive system. Thus it would be desirable to identify a low-cost method that would permit a system designer to attain a desired link budget in an optical communication network.